A field emission electron source is excellent in monochromaticity and capable of emitting an electron beam at high brightness, and used as an electron source, for example, of a scanning electron microscope or a transmission electron microscope at high resolution power. As the field emission electron source, while tungsten (W) has been used generally, since W has a large work function, electron tunnel probability is small and the amount of emitted current is small. Then, a field emission electron source using a hexaboride nanowire of low work function is proposed (for example, Patent Literature 1). Patent Literature 2 discloses a thermal electron source of heating a hexaboride to emit thermal electrons.